The major defense against nasally introduced respiratory infections are local immune responses and mucociliary clearance. One way to study the causes of failure of these defenses is to introduce a naturally occurring additional factor which is known to affect them. Malnutrition is the most powerful additional factor in much of the underdeveloped world; it interacts with respiratory infection to cause heavily increased mortality, especially in children under 4 years of age. The interactions are poorly understood. Both of the above defense mechanisms, and the effects of a single-factor nutritional deficiency on their function, will be studied in a laboratory model. In the domestic chicken we have established that vitamin A deficiency and Newcastle disease virus (NDV) interact to produce sustained high virus titers in nasal tissues, impairment of the mucociliary clearance mechanism, destruction of lymphocytes responsible for cell-mediated and humoral immune responses and defective regeneration of nasal mucous membranes. We propose to study by light and electron microscopy the mechanisms by which NDV and A-deficiency interact to produce breakdown of the integrity of the mucus blanket and its underlying epithelial cells. The structure of the mucus blanket will be examined by scanning electron microscopy of the two surfaces. T and B cells will be identified in sections of nasal tissue in normal and infected animals by staining with fluorescent sera, and the destruction of these cells will be followed in the normal and vitamin A deficient animal. These experimental laboratory findings may have direct application to ongoing field studies.